Spark plug



Jupe 18, 1940.

W. HARPER, JR

SPARK PLUG Filed June 16, 1937 Patented June 18, 1940 SPARK PLUG William Harper, Jr., Port Washington, N. Y., as-

signor to H. B. Motor Corporation, New York, N. Y., a corporation of New York Application June 16, 1937, Serial No. 148,447

7 Claims.

This invention relates to spark plugs and has the special object of providing a spark plug of increased durability and eiliciency for use in aviation engines.

5 Because of the extreme heat at which they must operate, spark plugs which have been made for use in aviation engines have differed from ordinary spark plugs inthat the central electrode has a stem of large diameter and a conical head,

iii while the outer electrode consists of a number of metal prongs lying close to this conical head. These prongs are ordinarily secured to the lower or inner end of the body of the plug, which is accordingly extended downwardly below the insull lation. The cylinder gases have direct access to the insulator through the open spaces between the prongs. I have found that. this conventional construction has a number of serious disadvantages. When the explosion pressure is develi oped in the cylinder, burning gases are projected directly against the insulation and tend to heat the insulation excessively. On the compression stroke, the mixture is projected directly against the hot insulation and this tends to cause preg ignition and detonation. Furthermore, thesurfaces providing the spark gap are exposed to direct heat of the hot gases and are not adequately cooled, so that they wear away rapidly.

In accordance with my invention, these disad- 30 vantages are eliminated by simple changes in constructed which cause a radical diierence in operation. In an aviation spark plug constructed in accordance with my invention, the prongs forming the outer electrodes in the conventional a5 construction are replaced by a thick shield of heat-conductingmetal surrounding the conical end of the inner electrode so as to serve as an outer electrode and formed to provide an enclosed chamber in which the insulation is protected 40 from direct access of the cylinder gases. Furthermore, means are provided for effectively conducting heat away from the spark gap surfaces.

The nature of my invention may best be understood from a detailed description of the construcv45 tion and operation of the spark plugs embodying it which are shown in the accompanying drawing, in which- Fig. 1 is a side view 0f such a spark plug, mainly in axial section; 50 Fig. 2 is an enlarged bottom or inner end view of the spark plug shown in Fig. l;

Fig. 3 is an enlarged section on the line 3-3 of Fig. l; and

Fig. 4 is a' partial axial section showing a modi- 55 ilcation.

(Cl. 12S- 169) The spark plug shown in Figs. 1, 2 and 3 has an inner electrode I0 provided with a large stem II' and a conical head I2, insulation I3 surrounding the stem of the electrode, and a metal body I4 having a tubular extension I5 surrounding the 8 insulation I3 and extending somewhat below lor beyond it. As thus far described, the construction is that of the conventional aviation spark plug. Instead of the four prongs customarily provided at the inner end of the extension I5 of the body, I provide at this point a thick -metal heat-conducting shield 20. The shield 2II substantially closes the inner end of the tubular extension I5 of the body, but contains passages hereinafter described.' The shield 20 l. may be made integral with the tubular extension I 5 or of a separate piece of metal as shown. The shield is most desirably made of nickel or tungsten-plated steel or other oxidation-resisting material of high heat conductivity. En

The shield 20 is made annular in form by a central opening 2i corresponding in shape to the circumferential surface of the conical head I2 and of a slightly larger diameter than this surface so as to provide a clearance space 22 which consti- 25 tutes the spark gap.

The shield 20 also contains a plurality of restricted passages or bores 25, each having a length many times as great as its diameter. The passages 25` are inclined upwardly and tangentially, 3o so that gas entering them from the cylinder is directed outwardly against the extension i5 of the body and moves spirally upward along the outer wall of the chamber provided by the extension I5 to the top of the chamber before it comes 35 in contact with the insulation I3. The passages 25 open into the central opening 2I through narrow grooves 2B which extend spirally along the surface of the central opening 2|. The passages 25 have diameters much larger than the width of o the clearance space 22. For convenience in con-` struction, the shield 2li may be formed with a circumferential recess 2'I providing a central projecting portion 28 on the outer side of the shield; but this is not essential.

The size and inclination of the passages 25 are such that they do not interfere with the ilow oi heat away from the conical spark gap surface of the shield. Heat from this surface is conducted radially outwardly through the heavy mass of metal forming the shield to the extension I5 of the body which conducts it to the cylinder wall.

An additional feature of my invention consists in providing for adequate heat conduction away from the sparking surface of the. inner electrode. u

This feature of the invention is, in the form shown, embodied in heat-radiating fins 3| formed on the body I4 above the nut portion 34 which engages the outer surface of the cylinder wall when the external thread 32 is screwed into the cylinder wall. and a neck 33 of reduced thickness between the fins and the nut portion 3l.

The operation of the spark plug which has been described is as follows:

n the compression stroke, the mixture from the cylinder enters the chamber within the extension I5 through the passages 25 and spirals upwardly against the outer wall of this cham ber before striking the insulation. Some mixture also enters through the clearance space 22, but this also is directed away from the insulation I3. The flow through the clearance space 22 is relatively slow except along the spiral slots 26. The rapid movement of the gas along these spiral slots by suction creates reduced pressure in the clearance space which is favorable to the jumping of the spark. The spark occurs in all parts of the clearance space except at the slots 26. The hot gas which enters through the passages 25 when the explosion pressure is developed follows the course described in connection with the mixture entering through the passages and so does not come into direct contact with the insulation until it has been cooled by the walls of the passages 25 and the outer metal wall of the chamber provided by the extension I5. During the exhaust stroke, the outward flow of the gas through the passages 25 and along the slots 26 by suction effects a thorough scavenging action in the clearance space 22.

Heat is effectively conducted away from both surfaces of the spark gap 22. Each surface is formedfon a massive piece of metal which conducts heat away from the surface. Heat is conducted from the outer spark gap surface radially outward through the shield and then through the extension I5 of the body to the cylinder wall. Heat is conducted from the inner spark gap surface through the stem of` the inner electrode to the threaded metal plug which is screwed into the body above the neck 33 andk then radiated from the fins 3I. The neck 33 chokes oi heat tending to flow from the cylinder wall to thecooling fins, and thus permits these fins to remain below the temperature of the cylinder wall so that they effectively conduct heat away from the central electrode.

In the form which has been described, all parts of the sparking surface of the conical head I2 are effectively shielded on direct access of radiant heat or hot gas from the cylinder except the part at the very end of the head. This part also of the head may be protected by the modified shield 20' shown in Fig. 4, which has a projection 30 coveringv the end of a central conical opening 2I' in the shield. While the modified construction shown in Fig. 4 slightly increases the life of the head of the central electrode, the form shown in Fig. 1 has the advantage of permitting direct observation of the width of the spark gap clearance space 22.

What I claim is:

1. A spark plug comprising the combination with a central electrode, an insulator therefor and a metal body having an extension surrounding the insulator, of a thick metal shield closing the end of said extension and containing a central opening providing a narrow clearance around the end portion of the central electrode and a passage of greater diameter than the width of said clearance space and inclined to direct incoming gases away from the insulator.

2. A spark plug comprising the combination with a central electrode, an insulator therefor and a metal body having an extension surrounding the insulator, of a thick metal shield closing the end of said extension and containing a central opening providing a narrow clearance around the end portion of the central electrode and a plurality of inclined tangential passages each of greater diameter than the width of said clearance space.

3. A spark plug comprising the combination with a central electrode having a conical head, an insulator and a metal body having an extension surrounding the insulator, of a thick metal shield closing the end of said extension and containing a central conical opening providing a narrow clearance space around said head and an inclined tangential passage.

4. A spark plug comprising the combination with a central electrode having a conical head, an insulator and a. metal body having an extension surrounding the insulator, of a thick metal shield closing the end of said extension and containing a central conical opening providing a narrow clearance space around said head and an inclined tangential passage openi ing linto said clearance space through a narrow slot.

5. A spark plug comprising the combination with a central electrode having a conical head, an insulator and a metal body having an extension surrounding the insulator, of a thick metal annulus closing the end of said extension and having a conical inner surface containing a plurality of spiral grooves.

6. A spark plug comprising an insulator, a metal body supporting the insulator and forming a casing providing an enclosed chamber surrounding the inner end of the insulator, said casing containing a central opening and a restricted passage so inclined that its axis does not intersect the insulator, and a central electrode extending into said central opening and having in said opening a slight clearance to provide a spark gap.

7. A spark plug having two electrodes providing cooperating spark gap surfaces each of which is formed on a massive piece of heat-conducting metal, a body having an external thread for screwing into .a cylinder wall and heat-radiating fins above the thread and formed to provide a heat-conducting path from the outer electrode to the threaded portion and to choke conduction of heat from the threaded portion to the fins, and means immediately adjacent and above said fins providing an effective heat-conducting path of large cross-section from the inner electrode to the ns.

\ WILLIAM HARPER, JR. 

